Sergey Doroshenko commented on ZOOKEEPER-704:

Dave, thanks for feedback,

Did you check http://wiki.apache.org/hadoop/ZooKeeper/GSoCReadOnlyMode ?

Approach described there is similar to what you've proposed: make server 
distinguish read-only and usual clients.
However, I was thinking that r-o client should go to read-only mode right after 
server it's tied to is partitioned, without trying to reconnect to majority. 
But your idea that client should try all servers first is definitely a better 

Also I think current behavior of ZooKeeper client should remain unchanged. 
I mean, there should be either new class for r-o client, or new functionality 
in current client which is explicitly triggered say by a flag passed to ctor. 
The idea is not to break code for current  users.

> GSoC 2010: Read-Only Mode
> -------------------------
>                 Key: ZOOKEEPER-704
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-704
>             Project: Zookeeper
>          Issue Type: Wish
>            Reporter: Henry Robinson
> Read-only mode
> Possible Mentor
> Henry Robinson (henry at apache dot org)
> Requirements
> Java and TCP/IP networking
> Description
> When a ZooKeeper server loses contact with over half of the other servers in 
> an ensemble ('loses a quorum'), it stops responding to client requests 
> because it cannot guarantee that writes will get processed correctly. For 
> some applications, it would be beneficial if a server still responded to read 
> requests when the quorum is lost, but caused an error condition when a write 
> request was attempted.
> This project would implement a 'read-only' mode for ZooKeeper servers (maybe 
> only for Observers) that allowed read requests to be served as long as the 
> client can contact a server.
> This is a great project for getting really hands-on with the internals of 
> ZooKeeper - you must be comfortable with Java and networking otherwise you'll 
> have a hard time coming up to speed.

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